Is it a good practice to have logger as a singleton?

Question

I had a habit to pass logger to constructor, like:

public class OrderService : IOrderService {
     public OrderService(ILogger logger) {
     }
}
         


        

Answer 1

A plain singleton is not a good idea. It makes it hard to replace the logger. I tend to use filters for my loggers (some "noisy" classes may only log warnings/errors).

I use singleton pattern combined with the proxy pattern for the logger factory:

public class LogFactory
{
    private static LogFactory _instance;

    public static void Assign(LogFactory instance)
    {
        _instance = instance;
    }

    public static LogFactory Instance
    {
        get { _instance ?? (_instance = new LogFactory()); }
    }

    public virtual ILogger GetLogger()
    {
        return new SystemDebugLogger();
    }
}

This allows me to create a FilteringLogFactory or just a SimpleFileLogFactory without changing any code (and therefore complying to Open/Closed principle).

Sample extension

public class FilteredLogFactory : LogFactory
{
    public override ILogger GetLogger()
    {
        if (typeof(ITextParser).IsAssignableFrom(typeof(T)))
            return new FilteredLogger(typeof(T));

        return new FileLogger(@"C:\Logs\MyApp.log");
    }
}

And to use the new factory

// and to use the new log factory (somewhere early in the application):
LogFactory.Assign(new FilteredLogFactory());

In your class that should log:

public class MyUserService : IUserService
{
    ILogger _logger = LogFactory.Instance.GetLogger();

    public void SomeMethod()
    {
        _logger.Debug("Welcome world!");
    }
}